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Water bound to various hydrous minerals can be transported deep into the mantle by slab subduction. Serpentine is one of most important hydrous minerals in the crust and shallow upper mantle. A partially serpentinized slab mantle limits the amount of water that can enter deep into the mantle. The partitioning of water between hydrous minerals and nominally anhydrous minerals (NAMs) is a crucial factor in controlling the physical properties and dynamics of slabs. Recent experiments on water partitioning have revealed that water strongly partitions to coexisting hydrous minerals. NAMs, such as olivine and its high-pressure polymorphs, have limited water content in water-undersaturated wet slabs. Metastable olivine wedges are not a feature of dry slabs, but can be of wet slabs that are not saturated with water. The transformation kinetics of the dry state, which generates deep-focus earthquakes and produces significant deformation in the slab, can work under wet slabs. Water bound to hydrous minerals is transported by the slab to the mantle transition zone and lower mantle. Hydrous minerals in stagnant slabs over 660 km depth release water as the slab warms, producing locally hydrated mantle transition zones and dense water-bearing magmas at the base of the upper mantle, and generating intraplate volcanism, which are referred to as the big mantle wedge model. Seismic scatterers are observed in the lower mantle at depths from 700 to 1900 km. These scatterers may be caused by water release at the top of the lower mantle by dehydration of hydrous minerals such as dense hydrous magnesium silicates. The shear instability due to the second order phase transformation from stishovite to CaCl2-type phase in hydrous aluminous SiO2 also causes the depth variation of seismic scatterers in the lower mantle. The high-pressure polymorphs of aluminous SiO2 contain a large amount of water more than 1 wt%, which can be important water carriers under lower mantle conditions. [ABSTRACT FROM AUTHOR]

The stishovite to post‐stishovite phase transition may modify the scattering of seismic waves by stishovite‐bearing rocks in the Earth's lower mantle. A series of continuous compression experiments on sintered polycrystalline stishovite was performed to study the effect of stress on the phase transition. The experimental results show that the phase transition shifts to lower pressures as the magnitude of deviatoric stress increases. Our results further show that the bulk modulus of sintered polycrystalline stishovite differs from that derived from single crystal measurements and decreases at the phase transition. In cold regions, such as subducted slabs, stresses may accumulate and shift the phase transition to a shallower depth. In hot regions with less stress, such as rising plumes, the phase transition is shifted to a greater depth. In addition, the phase transition may have varying seismic signatures depending on the behavior of the grain boundaries in mantle rocks and the micro‐stresses present in neighboring grains. Plain Language Summary: Stishovite and post‐stishovite are high‐pressure polymorphs of SiO2 that are stable in the Earth's lower mantle. The stishovite to post‐stishovite phase transition may affect the way seismic waves travel through mantle rocks. To detect SiO2 phases in the deep mantle, it is essential to investigate the depth of the phase transition and its effects on seismic wave propagation. Experiments were conducted to simulate the high‐pressure environment of the lower mantle and to study the phase transition of sintered polycrystalline stishovite aggregates, in which crystals are firmly connected to each other. Our study finds that the phase transition is sensitive to deviatoric stress, which is a measure of the directional stress difference. An increase in deviatoric stresses shifts the phase transition to shallower depths in the mantle. We further find that at pressures below the phase transition, sintered polycrystalline stishovite aggregates exhibit lower compressibility compared to stishovite powder and single crystals, but the compressibility increases abruptly at the phase transition, probably caused by grain‐grain interactions in the sintered samples. Therefore, the behavior of seismic waves will be different when passing through stishovite‐bearing rocks with different degree of grain‐grain interactions, particularly under conditions close to the phase transition boundary. Key Points: Continuous compression experiments were performed to study the post‐stishovite phase transition under different stress conditionsDeviatoric stresses shift the phase transition to a shallower depth in the lower mantleThe bulk modulus of sintered polycrystalline stishovite differs from that of stishovite powder and drops at the phase transition [ABSTRACT FROM AUTHOR]

We analyze teleseismic array data of deep and intermediate-depth earthquakes in the Fiji-Tonga-Kermadec region recorded at seismic networks at the west coast of US and Alaska with the aim of mapping seismic scatterers in the lower mantle and in the upper mantle transition zone beneath the Samoan hotspot. Short period and broadband waveform data are array-processed in order to detect coherent but anomalous phases in the P coda up to nearly 120 s after direct P waves and locate where they arrive from. Updating the data set reported in our previous study (Kaneshima and Helffrich, 2010; Kaneshima, 2018; Kaneshima, 2019), we examine nine scatterers near the hotspot, seven in the lower mantle and two in the transition zone, with the main focus on three lower mantle scatterers nearest to the hotspot at 1000 to 1200 km depths. We observe S-to-P waves generated from the three lower mantle scatterers for earthquakes with a wide range of hypocenters from 17°S to 24°S, while hardly any signs of corresponding P-to-P scattering waves are observed. The pulse shapes of the S-to-P waves from the three scatterers are simple and their pulse widths do not significantly differ from those of direct P waves when the signal quality is high enough. The geometry of the scatterers suggested by these observations is a steeply dipping sheet-like structure with the thickness less than several kilometers. The material consisting of the sheet needs to have a Vs value at least several percent lower than the surrounding mantle, as required by the amplitudes of S-to-P scattering waves relative to P, which is typically several percent. The low Vs anomaly of basaltic rocks relative to pyrolite under the shallow lower mantle conditions may be consistent with the observations (Wang et al., 2020). Seismic tomography models suggest that the nine scatterers tend to be located around the edge of a plume that ascends from the Pacific LLSVP to the Samoan hotspot. Numerical simulations of ascending plumes from a thermo-chemical pile at the bottom of the mantle show that the plumes tend to rise preferentially in the vicinity of the edge of the pile. Other simulations also show that blobs of recycled former oceanic crust subducting to the bottom of mantle are entrained into a plume and stretched vertically to a significant degree, consistently with the images suggested by geochemical studies of OIB. The scattering observations therefore may elucidate the structure of entrained basaltic blobs on their way of returning back to the surface hotspot. [Display omitted] • Seismic scatterers are found in the lower mantle and transition zone beneath the Samoan hotspot. • The scatterers represent compositional heterogeneities. • The scatterers tend to be located near the edge of a large low Vs body rising from the Pacific LLSVP. • The scatterers may represent basaltic materials entrained into the plume near the base of the mantle. [ABSTRACT FROM AUTHOR]

Tomographic images have shown that there are clear high-velocity heterogeneities to the north of the Pacific Anomaly near the core–mantle boundary (CMB), but the detailed structure and origin of these heterogeneities are poorly known. In this study, we analyze PKP precursors from earthquakes in the Aleutian Islands and Kamchatka Peninsula recorded by seismic arrays in Antarctica, and find that these heterogeneities extend ∼400 km above the CMB and are distributed between 30° and 45°N in latitude. The scatterers show the largest P-wave velocity perturbation of 1.0–1.2% in the center (160–180°E) and ∼0.5% to the west and east (140–160°E, 180–200°E). ScS–S differential travel-time residuals reveal similar features. We suggest that these seismic scatterers are the remnants of ancient subducted slab material. The lateral variations may be caused either by different slabs, or by variations in slab composition resulting from their segregation process. [ABSTRACT FROM AUTHOR]

Abstract: Among dense-hydrous magnesium silicates potentially transporting H2O into Earthʼs deep interior, phase D (MgSi2H2O6) exhibits the highest P–T stability range, extending into the lower mantle along cold slab geotherms. We have studied the compressibility and spin state of Fe in Al-bearing phase D up to 90 GPa using synchrotron X-ray diffraction and X-ray emission spectroscopy. Fe–Al-bearing phase D was synthesized at 25 GPa and 1400 °C with approximate composition MgSi1.5Fe0.15Al0.32H2.6O6, where nearly all of the Fe is ferric (Fe3+). Analysis of Fe-Kβ emission spectra reveals a gradual, pressure-induced high-spin (HS) to low-spin (LS) transition of Fe3+ extending from 40 to 65 GPa. The fitted equation of state for high-spin Fe–Al-bearing phase D results in a bulk modulus with pressure derivative . An equation of state over the entire pressure range was calculated using the observed variation in low-spin fraction with pressure and a low-spin bulk modulus of , derived from the data above 65 GPa. Pronounced softening in the bulk modulus occurs during the spin transition, reaching a minimum at 50 GPa (∼1500 km) where the bulk modulus of Fe–Al phase D is about 35% lower than Fe–Al-bearing silicate perovskite. Recovery of the bulk modulus at 50–65 GPa results in a structure that has a similar incompressibility as silicate perovskite above 65 GPa. Similarly, the bulk sound velocity of Fe–Al phase D reaches a minimum at ∼50 GPa, being about 10% slower than silicate perovskite. The potential association of Fe–Al phase D with subducted slabs entering the lower mantle, along with its elastic properties through the Fe3+ spin transition predicted at 1200–1800 km, suggests that phase D may provide an alternative explanation for small-scale mid-lower mantle seismic scatterers and supports the presence of deeply recycled sediments in the lower mantle. [Copyright &y& Elsevier]

Abstract: Lower mantle seismic scatterers are often interpreted in terms of subducted basaltic crust, which should contain free-silica phases that are seismically fast. However, free-silica phases are unstable in a peridotite lower mantle, as they react with ferropericlase (magnesiowustite) to form silicate perovskite. Nonetheless, conservative estimates and recent measurements of diffusion coefficients in silicate perovskite suggest that free-silica phases may persist as polycrystalline armored relics in lower mantle peridotite at length scales equivalent to those observed for seismic scatterers. Simple numerical simulations also suggest that polycrystalline silica may persist within subducted metabasalts. [ABSTRACT FROM AUTHOR]

Abstract: Images of the scattering field near the core–mantle boundary (CMB) beneath the Caribbean were obtained from migration processing of PKIKP precursors recorded by a broadband seismic array in the southern Caribbean from earthquakes in the Western Pacific. Strong seismic scatterers were found to be concentrated near the CMB north of Colombia and are surrounded by high velocity anomalies associated with the subducted Farallon plate. The relative location between the imaged scatterers and the surrounding high velocity anomalies leads us to speculate that the observed seismic scatterers are remnants of the subducted slab. As the Farallon plate penetrated into the lower mantle, segregation allowed its basaltic crust to descend more quickly and to pool in front of the major portion of the subducting slab. The ponding oceanic crustal material could be further collected (or bulldozed) by the residual motion of the subducting plate, forming a localized region with strong seismic scatterers. [Copyright &y& Elsevier]